Method of controlling the operation of an approach system of a paper machine or the like web formation apparatus

ABSTRACT

The present invention relates to a method and apparatus for controlling the operation of an approach system of a paper machine or the like web formation apparatus. Especially preferably the method of controlling the operation of an approach system according to the invention is applicable to be used in connection with paper and paperboard machines as well as in connection with various apparatuses performing non-woven webformating.

This application is the U.S. national phase of international applicationPCT/F100/00709 filed Aug. 22, 2000, which designated the U.S.

The present invention relates to a method of controlling the operationof an approach system of a paper machine or the like web formationapparatus. Especially preferably the method of controlling the operationof an approach system according to the invention is applicable to beused in connection with paper and paperboard machines as well as inconnection with various apparatuses performing non-woven webformating.

Almost all prior art paper machine approach systems, which are welldescribed in, e.g. U.S. Pat. No. 4,219,340, comprise the followingcomponents: a mixing tank, a feed/mixing pump, a centrifugal cleaningplant, a gas separation tank, a head box feed pump, a head box screen, apaper machine head box and white water trays. Said components are placedin connection with the paper machine and arranged to operate as follows:The fiber material used for paper making and fillers which are dilutedwith so-called white water obtained from the paper machine, mostly fromthe wire part thereof, are dosed into the mixing tank often referred toas wire pit and located at the bottom level of the mill. By means of thefeed/mixing pump also located at the bottom level of the mill, the fibersuspension is pumped from the mixing tank to the centrifugal cleaningplant located usually at the machine level of the mill, i.e. thelocation level of the paper machine, or, as in said patent, above it. Bymeans of pressure created by said mixing pump, the fiber suspensionaccepted by the centrifugal cleaning plant is further conveyed to thegas separation tank located at a level above the machine level. From thegas separation tank the fiber suspension, wherefrom gas has been removedas thoroughly as possible, flows to the head box feed pump located atthe bottom revel of the mill, which pump pumps the fiber suspension tothe head box screen (not shown in said US patent) also located at thebottom level of the mill, wherefrom the fiber suspension flows to themachine level into the head box of the paper machine.

In order to operate, the gas separation tank described both in said U.S.Pat. No. 4,219,340 and in form of a somewhat newer modification in U.S.Pat. No. 5,308,384 requires a vacuum system most often comprising avacuum pump, most commonly a so-called liquid ring pump, located at thesame level as the gas separation tank, and a drop separator for removingliquid drops possibly present in the gas withdrawn by the vacuum pump.The actual gas separation tank is traditionally a large essentiallyhorizontal tank into which the fiber suspension coming from thecentrifugal cleaning plant is sprayed via separate injection pipes. Thepurpose of the spraying is to give the gas in bubble form a possibilityto leave the fiber suspension as early as this stage. In most cases, thegas separation tank is further provided with an intermediate wall, aso-called overflow weir meant for stabilizing the surface level of thefiber suspension in the tank, although there are gas separation tankswith no overflow. The objective of keeping the surface level constant isto ensure a constant inlet pressure for the head box feed pump, at thesame time ensuring a constant flow of paper pulp to the head box. Inother words, the amount of fiber suspension fed to the gas separationtank via the centrifugal cleaning plant is always somewhat greater thanrequired by the head box. The excess fiber suspension is led via theoverflow weir usually to the other end of the gas separation tank,wherefrom a return pipe leads to the mixing tank. Fiber suspension to bepumped to the head box is obtained via a discharge outlet arranged atthe bottom of the head box and led to the head box feed pump. Variousgas separation tank solutions are disclosed in e.g. U.S. Pat. Nos.5,236,475, 4,478,615, 4,455,224, 3,538,680, 2,717,536, 2,685,937 and2,642,950, the three last mentioned of which deal with a gas separationtank without overflow.

U.S. Pat. No. 2,717,536 discusses a gas separation apparatus wherein thefiber suspension flow coming from the centrifugal cleaning plant is ledto a gas separation tank having no overflow weir but wherein the surfacelevel is kept constant by means of a surface level transducer and a feedpump flow regulation valve controlled by said transducer. Further saidpublication presents the location of the gas separation tank at themachine level, i.e. the same level as the paper machine head box.

U.S. Pat. No. 2,685,937 also discloses a gas separation tank with nooverflow. In the solution of said patent, there is a float arranged inthe gas separation tank, which float follows the changes of the surfacelevel of the fiber suspension. The movements of the float have a directeffect on the fiber suspension flow being fed into the tank, as theyregulate the amount of fiber suspension being fed to the tank via theinjection pipes.

Said prior art apparatuses have some disadvantages of which e.g. thefollowing problems are worth mentioning.

Firstly, the surface level of the gas separation tank controlled byeither an over-flow or various float solutions or other devices directlyfollowing the surface level does naturally remain constant, but thatdoes not lead to what the actual purpose of the surface level regulationis, i.e. a constant inlet pressure of the head box feed pump. A reasonfor this Is that the density of the fiber suspension being pumpedtogether with the surface level determine the inlet pressure. Saiddensity, in turn, is effected by e.g. the filler content and gas contentof the fiber suspension. Despite the fact that the filler content of thefiber suspension should be as constant as possible, there are somefluctuations in that. The fluctuations in the density are mostly causedby the gas content of the fiber suspension, which gas content may, inthe worst case, vary to the extent of several per cents. Such greatchanges in the density of the fiber suspension lead to fluctuations inthe pulp amount pumped by the head box feed pump, which again isreflected in fluctuations in the thickness of the final product.

In addition to that, prior art apparatuses are not capable of quicklyresponding to problems caused by e.g. a change in the speed of themachine. According to prior art, attempts were made to solve theseproblems in a way presented in the block diagram of FIG. 2, whichdescribes a situation where the speed of the paper machine is eitherincreased (the right side of the Figure) or decreased (the left side ofthe Figure), i.e. the production of the web formation apparatus ischanged. In a prior art system, the head box slice flow is naturallychanged first, as the production of the machine is controlled by meansof it, whether calculated in terms of basis weight of the product or intons produced by the machine. The starting point is to keep both thehead box pressure and the product grammage constant despite changes inthe speed of the machine. By means of prior art regulation system thisis done so that as the speed of the paper machine increases the sliceopening is widened in such a way that a constant pulp amount inproportion to the speed of the wire is continuously flowing from theslice opening (assuming that the head box pressure is constant). Whenthe regulation system senses the widening of the head box slice openingin form of pressure decrease in the head box, the pressure will beincreased by increasing the output of the head box feed pump. This inturn results in the lowering of the surface level in the gas separationtank, whereby the regulation system makes the mixing pump feed more pulpinto the gas separation tank and the surface level of the tank returnsto normal. A regulation arrangement of this type causes various pressurefluctuations in the approach system. Firstly, in order to keep the headbox pressure constant as the surface level in the gas separation tank aswell as the inlet pressure of the head box feed pump decrease, thecapacity of the mixing pump is increased. When the regulation hasreached the mixing pump, the mixing pump increases the feed of the gasseparation tank, whereby the surface level thereof starts to rise. Thiscauses the head box pressure to increase, which in turn leads todecreasing the capacity of the head box feed pump in order to stabilizethe pressure. As the surface level in the gas separation tank hasreached its set value, the regulation system guides the mixing pump todecrease the flow, which generates a new pressure effect in the headbox. This time the head box pressure leaps downwards, because the headbox feed pump has decreased its capacity to correspond to the risingsurface of the gas separation tank. As the surface level no more rises,the inlet pressure of the head box feed pump does not rise either. Theregulation system handles with this situation by increasing the capacityof the head box feed pump in order to increase the head box pressure toits set value. In practice said development brings forth the danger thatthe whole production produced during said regulation will turn to broke,as fluctuations in the head box pressure are directly reflected in basisweight fluctuations of the production. In a corresponding way, theconsequent effects of decreasing the speed of the paper machine are seenin the left part of FIG. 2.

The basic reason for the problems is that various regulation operationsare performed in delay, which means that obvious changes have alreadytaken place either in the head box pressure, the surface level in thegas separation tank or both. In such a case compensating for these callsfor changes in the opposite direction, the compensation of which inprior art is further performed in delay, which naturally leads to asituation where reaching the balanced stage takes an unreasonably longtime.

Characterizing features of the method of controlling the approach systemof a paper machine or the like according to the invention, which solvee.g. said problems, are disclosed in the appended claims.

In the following, the method of controlling the operation of theapproach system of a paper machine or the like according to theinvention are explained in more detail with reference to the appendedfigures, of which

FIG. 1 mainly illustrates a prior art solution disclosed in U.S. Pat.No. 4,219,340,

FIG. 2 is a block diagram of a prior art system for regulating the headbox pressure,

FIG. 3 illustrates in form of a block diagram a regulation system for apaper machine approach system according to a preferred embodiment of theinvention, and

FIG. 4 illustrates an apparatus arrangement being applied in theembodiment of FIG. 3.

The prior art paper machine approach system illustrated in FIG. 1comprises a mixing tank/wire pit 10, a feeding/mixing pump 12, acentrifugal cleaning plant 14, a gas separation tank 16, a head box feedpump 18, a head box screen 20, a paper machine head box 22 and whitewater trays (not shown). Said components are placed in connection withthe paper machine 24 and arranged to operate as follows. The fibermaterial used in papermaking, which may comprise fresh stock, secondarypulp or broke, and so-called fillers, which are diluted with so-calledwhite water obtained from the paper machine, mostly from its wiresection, are dosed into the mixing tank 10, which tank may also be awire pit wherein the white waters are collected and which is located atthe bottom level of the mill, to produce paper pulp. By means of amixing pump 12 also located at the bottom level of the mill said paperpulp is pumped from the mixing tank 10 to a centrifugal cleaning plant14 usually located at the machine level K of the mill, i.e. at thelocation level of the paper machine 24. The paper pulp accepted by thecentrifugal cleaning plant is further conveyed by means of pressurecreated by mixing pump 12 into the gas separation tank 16 located at alevel T above the machine level. From the gas separation tank 16 theessentially gas-free paper pulp, wherefrom gas has been removed asthoroughly as possible, flows to the head box feed pump 18 located atthe bottom level of the mill, which head box feed pump pumps the paperpulp to the headbox screen 20 also located at the bottom level,wherefrom the accepted paper pulp flows to the machine level K into thehead box 22 of the paper machine 24.

The operation of the gas separation tank 16 requires a vacuum system 17,most commonly comprising a vacuum pump, in most cases a so-called liquidring pump located at the same level as the gas separation tank 16, and adrop separator, by means of which drops of liquid possibly present inthe gas withdrawn by the vacuum pump and flowing towards the vacuum pumpare removed from the gas. The actual gas separation tank 16 istraditionally a large-sized essentially horizontal tank, in to the innerside of which the paper pulp coming from the centrifugal cleaning plant14 is injected via separate injection pipes. The purpose of theinjection is to allow the gas in bubble form a possibility to leave thepaper pulp at this stage already. The gas separation tank 16 is furtherprovided with an intermediate wall, a so-called overflow weir, theobjective of which is to keep the surface level P of the paper pulpconstant in tank 16. This is done in order to ensure a constant inletpressure for the head box feed pump 18. In other words, the amount ofpulp fed from the mixing tank 10 via the centrifugal cleaning plant 14to the gas separation tank 16 is always to some extent greater thanrequired by the head box 22. The excess paper pulp is led via theoverflow weir usually to the other end of the gas separation tank 16,wherefrom a return pipe 34 leads to the mixing tank 10. The paper pulpto be pumped to the head box 22 is obtained via a discharge outletarranged at the bottom of the gas separation tank 16 and a dischargepipe 36 connected therewith, and lead to the head box feed pump 18.

The block diagram of FIG. 2 illustrates a prior art system for keepingthe head box pressure constant. The block diagram of FIG. 2 discloses asituation where the speed of the paper machine is decreased (left sideof the figure) or increased (right side of the figure). The procedure isexactly the same in any other grade change situation, too. Firstly, whenincreasing the speed of the paper machine, i.e. increasing theproduction, it is possible to change the slice flow of the head box 22either by opening the slice of the head box 22 or by increasing the headbox pressure to correspond to the increase of the speed of the papermachine. In most cases increasing the head box pressure compensates forthe increase in the speed of the machine. A regulation system of priorart thus requires that the speed of the machine corresponds to a certainhead box pressure value, whereby the increased speed of the machinewould require higher pressure in the head box compared to earlier value.As a matter of fact, the situation would be just the same, if the headbox pressure would start to decrease in a constant running situation. Inthat case the regulation system naturally guides the feed pump 18 of thehead box 22 to increase the head box 22 feed. This in turn results inlowering of the surface level in the gas separation tank 16, whereby theregulation system makes the mixing pump 12 feeding paper pulp to the gasseparation tank 16 feed more pulp into the gas separation tank 16 andthe surface level of the tank 16 returns to its previous height. Thiskind of regulation arrangement generates many kinds of pressurefluctuations in the approach system. Firstly, in order to keep thepressure in the head box 22 constant when the surface level in the gasseparation tank 16 descends and the inlet pressure of the feed pump 18of the head box 22 decreases at the same time, the capacity of the feedpump 18 is increased. When the control function of the regulation systemhas reached the mixing pump 12, the mixing pump 12 increases the feed tothe gas separation tank 16, whereby the surface level in the tank 16starts to rise. As a result, the pressure in the head box 22 increases,which in turn leads to decreasing the capacity of the feed pump 18 ofthe head box 22 in order to stabilize the pressure. And, when thesurface level in the gas separation tank 16 has reached its set value,the regulation system guides the mixing pump 12 to decrease the flow,which generates the following pressure effect on the head box 22. Thistime the pressure in the head box 22 leaps downwards, as the feed pump18 of the head box 22 has reduced its capacity to correspond to therising surface of the gas separation tank 16. The regulation systemhandles the situation by increasing the capacity of the feed pump 18 ofthe head box 22 to increase the pressure of the head box 22 to its setvalue. In practice said development results in driving the wholeproduction obtained during said regulation to broke, as pressurefluctuations of the head box are directly reflected in fluctuations ofthe grammage of the production.

The block diagram of FIG. 3 illustrates a way to handle the head boxpressure regulation and the gas separation tank surface level regulationaccording to a preferred embodiment of the invention in such a way thatdisadvantageous pressure fluctuations are avoided. The starting point inthe example of FIG. 3, just as in the example of FIG. 2, is a situationwhere the pressure in the head box changes (in this example the pressuredecreases) or it is assumed to change due to either change of grade,change in the speed of the paper machine or to some other reason. Theregulation system according to the invention may receive a signal onpressure decrease from several various sources.

One possibility is to utilize information from the pressure transducerof the head box. Another possibility is to monitor the operating pointof the head box feed pump. As the pressure in the head box changes, theoperating point of the head box feed pump changes, too. To put itdifferently, although the input power of the pump or the torque doesremain constant, the output of the pump changes as the rotational speedchanges. Or, if the rotational speed is kept constant, the change in thehead box pressure changes the power requirement of the pump.

A decrease in the head box pressure, e.g., directly results in adecrease in the back pressure of the pump, which leads either to anincrease in the output of the pump at constant capacity or to a decreasein the input power of the pump at constant rotational speed. Thus apressure change in the head box reflects in a change in the head boxfeed pump flow. In other words, at a certain capacity of the pump eachoutput value of the pump corresponds to a certain slice opening of thehead box and a certain head box pressure. When the output of the pumpchanges at said capacity, it indicates that a change has taken place inthe head box pressure and/or the slice opening. In that case, as thepressure in the head box decreases, the regulation system increases thecapacity of the mixing pump. That is, the mixing pump attempts to keepthe gas separation tank surface level constant and the head box feedpump attempts to keep the head box pressure constant. As the effect ofthe mixing pump reflects on the surface level of the gas separation tankrelatively slowly and because the surface level of the gas separationtank in turn reflects on the input pressure of the head box feed pumprelatively slowly, delays are necessary between various regulationoperations. These delays and set values of various regulation operationsare adjusted in the control and regulation system so that the surfacelevel of the gas separation tank and the pressure in the head box remainstable.

In practice the following initial data are needed for practising themethod of regulating the head box pressure and controlling the surfacelevel of the gas separation tank according to the regulation system ofthe present invention.

The surface level of the gas separation tank, the limit values thereofand the direction of change in the surface level.

The output i.e. capacity of the headbox feed pump

A neural network or some other indirect measuring method based onprogrammed calculating, i.e. a soft sensor, included in the regulationsystem determines the output of the pump e.g. from the performance chartof the pump on the basis of the rotational speed of the pump and thepressure difference

The output i.e. capacity of the mixing pump

A neural network or other “soft sensor” included in the regulationsystem determines the capacity utilizing said initial data

The head box pressure.

The regulation system handles or utilizes the predetermined data asfollows. When the regulation system senses the surface level of the gasseparation tank, i.e. the output of the web formation apparatus, tochange slowly, it checks whether it is necessary to change theoperational mode of the mixing pump and if necessary guides the head boxfeed pump to compensate for the change of, the inlet pressure, i.e. theoutput of the web formation apparatus, so that the feed pressure of thehead box remains constant. E. g. when the surface level descends, theregulation system smoothly and slowly increases the rotational speed ofthe head box feed pump, whereby the decreasing inlet pressure caused bythe decreasing surface level is compensated for by the slowly increasingoutput of the head box feed pump, which output increases the head boxpressure in the same proportion. If the velocity of the change in thesurface level of the gas separation tank is slow enough, the regulationsystem does not consider it necessary to change the operational mode ofthe mixing pump and thus does not guide the mixing pump to increase thefeed of the gas separation tank, as it is possible that the surfacelevel may return to normal by, itself. Only when the surface levelapproaches the limit value, the regulation system guides the mixing pumpto respond to said change. That is, the regulation system according tothe invention allows the surface level of the gas separation tank setfreely inside certain maximum and minimum limits.

In some cases it may also be considered necessary that the regulationsystem guides the mixing pump proactively so that the output of themixing pump is changed proactively in relation to the change of the feedpump output. This is done in order to take into account the delay, whichis caused by the pipeline between the mixing pump and the gas separationtank and the possible centrifugal cleaning plant. In that case, in anoptimal situation, the result is that the surface of the head box doesnot change at all when the slice flow i.e. the output of the webformation apparatus changes.

If there is a need to change the feed of the mixing pump to return thelevel of the gas separation tank to its set value i.e. approximately inthe middle between the limit values, the regulation system, depending onthe direction of change in the mixing pump feed, guides the feed pumpeither to decrease or increase the feed to the head box. If the surfacelevel in the gas separation tank e.g. attempts to decrease below itsminimum limit, the regulation system guides the mixing pump to increasethe feed to the gas separation tank. At the same time the regulationsystem is getting ready to decrease, after a certain delay, the outputof the head box feed pump i.e. in practice the pressure effect of thefeed pump on the head box. The reason for this is that the rising levelof the gas separation tank increases the inlet pressure of the feedpump, which in itself already increases the head box pressure.

By means of fuzzy logic applied to the surface level regulation of thegas separation tank the overflow may be omitted from the tank. Accordingto FIG. 3, the surface level regulation may actually be practiced inthree ways. The first method is to guide the operational mode of themixing pump, i.e. the change of the capacity, by means of fuzzy logicand the change in the surface level of the gas separation tank. A secondway is to further improve the surface regulation by means of feedforwardcontrol, whereby it is possible to determine the flow change from theperformance chart of the pump by means of a neural network on the basisof the changed rotational speed and pressure difference of the head boxfeed pump. This data on the change in the flow from the gas separationtank is transferred via a flow control loop or fuzzy logic directly tothe mixing pump, the rotations of which are changed utilizing neuralnetwork calculating (the performance chart and the pressure differencedata are needed in this) or by means of another “soft sensor”.Naturally, other flows entering the gas separation tank have been takeninto account. In practice the feedforward control is understood to meanthat the capacity of the head box feed pump is determined by means of aneural network and said capacity is required from the mixing pumpaugmented with possible reject flows which are separated in the processprior to the feed pump. The starting point is, of course, that theportion or amount of said reject flows is known. When various delays invarious parts of the process are taken into account in this feedforward,the pump feeds and changes in them may be timed to start in such a waythat in practice neither the surface of the gas separation tank nor thehead box pressure changes from their set values (the set value maynaturally change e.g. in a situation described in the example above).

Thus, fuzzy logic is actually not needed to minimize the head boxpressure fluctuations, but mostly to control the surface level of thegas separation tank. A neural network or another “soft sensor” assistsfuzzy logic in the feedforward of said regulation.

In a grade change situation, when the production of the machine ischanged and the head box pressure may also change remarkably, theregulation system preferably changes the head box pressure stepwise. Inthat case the regulation system starts to change the output of both themixing pump and the head box feed pump essentially simultaneously,naturally taking into account said delays.

It may even be thought that the operator of the production machineinforms the regulation system on the grammage of the desired finalproduct, after which the regulation system controls the rest of thegrade change optimizing the stages needed in that procedure. Inpractice, the required slice opening and head box pressure for eachpossible grammage have been fed in the regulation system beforehand.When the regulation system sees how much the slice opening and/or headbox pressure has to be changed, it operates according to presetprogramming, either performing in one single stage both the change ofthe slice opening and the change of the head box pressure oralternatively changing one or both of these in two or more stages. Theregulation system itself functions as already presented above.

Further, the presence or non-presence of the centrifugal cleaning plantin the approach system presents one additional nuance for the operationof the regulation system. If the approach system is devoid of acentrifugal cleaning plant, the regulation system operates as describedabove. If the approach system is provided with a centrifugal cleaningplant, the regulation system has to be capable of taking it into accountin some way. Actually the only thing worth taking into account about anexisting centrifugal cleaning plant is its reject flow. In other words,the centrifugal cleaning plant does not let all incoming material topass into the gas separation tank, but some of the material is led awayfrom the short circulation. There are several ways for taking thecentrifugal cleaning plant into account. One way is to always obtainfrom the centrifugal cleaning plant a constant reject flow despite theflow that enters the plant. In such a case it is easy for the regulationsystem to subtract from the mixing pump feed the portion that is passedto reject from the centrifugal cleaning plant and use the difference asthe initial value for subsequent measures. Another way is to always takea proportionally equal part of the flow to the reject flow. In this casethe procedure is in fact the same as already stated except that the trueflow amount passing to the gas separation tank is received bymultiplying the mixing pump feed by the accept flow ratio of thecentrifugal cleaning plant (e.g. 0.97). A third method is to separatelydetermine the reject flow amount, whereby the amount of material goingto the gas separation tank is calculated by subtracting the reject flowamount from the mixing pump feed.

FIG. 4 illustrates a solution according to a preferred embodiment of theinvention, wherein white waters from the paper machine 124 are led tothe white water tank 110, in the bottom part of which fiber suspensionand various fillers are combined to form paper pulp. From the whitewater tank 110, the paper pulp is conveyed by means of mixing pump 112into the centrifugal cleaning plant 114 and from there further to thegas separation tank 116 and further by means of the head box feed pump118 to the paper machine head box 122 as known from prior art. What isnew in the embodiment of the figure is the surface level control of thegas separation tank 116, which is effected neither by means of prior artoverflow nor by means of a system utilizing a float. As the purpose ofregulating the surface level of the gas separation tank 116 is to ensurethat the pressure of the paper pulp in the paper machine head boxremains as exactly constant as possible, the starting point for thesurface level control according to the invention is that the level ofthe paper pulp is allowed to vary within certain limits in the gasseparation tank 116 and the regulation of the pressure in the head box122 is effected by controlling the operation of the pumps 112 and 118. Asecond new method is a new type of way to ensure a constant pressure inthe head box. The pressure in the head box 122 is monitored by apressure transducer 148, the pressure impulse from which is registeredand led to the control unit 150 of the regulation system, which controlunit attempts to keep it constant. According to a preferred embodimentof the invention, it is effected primarily so that the feed pump 118 ofthe head box 122 is controlled on the basis of the impulse from saidpressure transducer 148 by means of the control unit 150 of theregulation system. When the pressure impulse indicates a tendency ofpressure decrease, the regulation system makes the head box feed pump118 increase its feed, whereby the pressure in the head box 122normalizes and the liquid level in the gas separation tank 116 descends.Accordingly, when the pressure increases, the control unit makes thehead box feed pump 118 decrease its feed, whereby the pressure in thehead box 122 is allowed to decrease and the level in the tank 116 rises.That is, the surface level in the gas separation tank 116 is allowed tovary to some extent.

Depending on the velocity of the change in the gas separation tank 116surface level, it is also possible to increase or decrease the feed,i.e. in practice the rotational speed, of the mixing pump 112 feedingpaper pulp to the gas separation tank 116. In other words, if thesurface level of the tank 116 changes very slowly, the central unit onlymonitors it. Naturally, to a certain limit. And if the surface leveldescends rapidly, the central unit makes the mixing pump 112 increasethe feed, i.e. change its operational mode so as to compensate for thedecrease of the gas separation tank 116 surface level. A correspondingregulation measure, i.e. a change of the operational mode of the mixingpump, but in an opposite direction, is naturally performed also when thesurface level in the tank 116 rises.

Naturally it is also possible to “teach” the regulation system,primarily the control unit, to more versatile operation, i.e. the speedof change of the pressure impulse from the pressure transducer 148 maybe arranged to guide the feed of mixing pump 112, too, and preferablyalso the rate of change of the feeding speed. Naturally, if it is notdesired to control the mixing pump 112 directly, it is possible toarrange a valve 154 in pipe line 152 between the mixing pump 112 and thegas separation tank 116, the operation of which valve is controlled(shown in dash line) also by means of the control unit on the basis ofimpulses from the pressure transducer 148.

Further, it is possible and in a certain situation also necessary toconnect a vacuum system 126 of the gas separation tank 116 to theregulation system i.e. more exactly said to the control unit. This maybe utilized e.g. when the stock level in the gas separation tank tendseither to descend too low or rise too high. When the level descends toolow, measured e.g. by means of a float device or some othercorresponding arrangement, the control unit gives a command to increasethe vacuum level of tank 116 and at the same time to increase the feedof the mixing pump 112, or alternatively the opening of valve 154,whereby the pressure transducer 148 controls that the inlet pressure ofthe feed pump 118 of the head box 122 remains constant. As the surfacelevel in the gas separation tank approaches the optimum value, thevacuum level and the feed of the mixing pump 112, alternatively theopening of valve 154, is gradually decreased under control of thepressure transducer 148, until the normal stage has been reached.Further, in a situation where an impulse from a transducer located onthe head box 122 or the wire part of the production machine indicates aneed to change the feed of the head box 122, it may be performed so thatthe control unit 150 interprets the incoming control impulse so that itsends a command to first change the feed of the gas separation tank 116accordingly (by means of the mixing pump 112 and/or the valve 154),whereafter it with a certain delay further sends a command to change thefeed of the feed pump 118 of the head box 122. The impulse resulting inthis change of the feed of the head box feed pump may be received e.g.from pressure transducer 148.

The pump 112 is preferably a propeller pump, because the feed pressurerequired from said pump in this application is not very high.Accordingly, the vacuum pump of the gas separation tank 116 ispreferably one of the High Speed vacuum pumps produced by Ahlstrom PumpsCorporation, the level of vacuum of which may be regulated by changingthe rotational speed of the pump. It is, of course, also possible to usethe older water ring pumps, the vacuum level of which may be regulatedby means of a valve.

Whether the approach system in question is one provided with acentrifugal cleaning plant, or one in which the centrifugal cleaning isarranged in an earlier stage for each pulp fraction separately,or onewithout any centrifugal cleaning, it is characterized by the tendency toplace the apparatuses at one and the same level, i.e. the machine level,if possible (if allowed by principles of physics). It has to be noted,though, that the head box feed pump usually may not be located at thesame level with the gas separation tank, as the underpressure in the gasseparation tank is so high that even a minor suction from the directionof the head box feed pump would generate cavitation resulting in theboiling of the water in the tank. For said reason, the head box feedpump has to be taken somewhat lower than the gas separation tank,whereby cavitation and the resulting boiling may be avoided.

As noticed from the above, a new type of paper machine approach systemhas been developed, which eliminates many prior art weaknesses anddisadvantages and solves problems which have been hampering the use ofprior art approach systems. Nevertheless, It has to be noted from theabove that the individual novelties presented above may be appliedseparately and not necessarily in the same connection as they have beenpresented above. Thus it is e.g. possible and wholly in accordance withthe invention to totally omit the centrifugal cleaning plant from theembodiment of FIG. 4.

What is claimed is:
 1. A method of controlling the operation of anapproach system of a web formation apparatus, in which said methodcomprises: a. forming a pulp from white water, fiber suspension andfillers, b. feeding said pulp by a mixing pump into a gas separationtank, c. separating gas from said pulp in the tank to produce reducedgas pulp, d. feeding reduced gas pulp to a bead box of the web formationapparatus, and e. changing the feed of the reduced gas pulp to the headbox as production changes in the web formation apparatus, and the changein the production of the web formation apparatus initiates a regulationsystem of the approach system, which regulation system essentiallysimultaneously checks a need for changing an operational mode of themixing pump, initiates a change of the operational mode of the mixingpump according to said need, and both guides and regulates a head boxfeed pump.
 2. A method according to claim 1 wherein the regulationsystem controls both a pressure of the head box and a surface level ofthe gas separation tank.
 3. A method according to claim 1 wherein anoperating point of the feed pump and an operating point of the mixingpump are changed essentially simultaneously.
 4. A method according toclaim 1 wherein a change of head box pressure is readable from a changeof an operating point of a head box feed pump, whereby said change ofthe operating point of head box feed pump initiates a control functionof the regulation system.
 5. A method according to claim 2 wherein asurface level of the gas separation tank is controlled by a controlfunction of the regulation system initiated by a change of the head boxpressure to a control function of the regulation system.
 6. A methodaccording to claim 5 wherein the regulation system guides simultaneouslythe feed pump and the mixing pump so that a pressure in the head boxremains constant and a surface level in the gas separation tank remainsconstant or changes in a controlled manner.
 7. A method according toclaim 1 wherein the regulation system changes at least an output of thehead box feed pump to keep a pressure in the, head box constant, and asurface level variation in the gas separation tank is monitoredsimultaneously and measurements are taken to control the surface levelof pulp in the gas separation tank.
 8. A method according to claim 1wherein a pulp surface level is allowed to change slowly and temporarilyin the gas separation tank without changing the feed of the gasseparation tank.
 9. A method according to claim 1 wherein when apressure of the head box changes slowly, and the change of the pressureis compensated for by changing a capacity of the head box feed pump,whereby a surface level of the gas separation tank is allowed to change.10. A method according to claim 1 wherein when a pressure of the headbox changes fast, the change of the pressure is compensated for bychanging essentially simultaneously a capacity of the head box feed pumpand a capacity of the mixing pump.
 11. A method as in claim 1 whereinthe web formation apparatus is a paper formation machine or a paperboard formation machine.
 12. A method as in claim 1 wherein said mixingpump is upstream in a flow of the pulp from the gas separation tank, andthe head box feed pump is downstream of the tank and upstream of thehead box in a flow of the reduced gas pulp.
 13. A method as in claim 1wherein the feed of the reduced gas pulp is a feed rate of the reducedgas pulp.
 14. A method of controlling the operation of an approachsystem of a web formation apparatus, in which said method comprises: a.forming a pulp from white water, fiber suspension and fillers; b.feeding said pulp by a mixing pump into a gas separation tank; c.separating gas from said pulp in the gas separation tank to yieldreduced gas pulp; d. feeding the reduced gas pulp into a head box of theweb formation apparatus; and e. changing the feed of the reduced gaspulp to the head box as production changes in the web formationapparatus, wherein the chance in the production of the web formationapparatus initiates a regulation system of the approach system, whichregulation system essentially simultaneously checks a need for changingan operational mode of the mixing pump, initiates a change of theoperational mode of the mixing pump according to said need, and bothguides and regulates a head box feed pump, wherein an operating point ofthe mixing pump is changed in anticipation of changing an operatingpoint of the feed pump so that a surface level in the gas separationtank located between said pumps remains essentially constant or changesin a controlled manner.
 15. A method of controlling the operation of anapproach system of a web formation apparatus, in which said methodcomprises: a. forming a pulp from white water, fiber suspension andfillers, b. feeding said pulp by a mixing pump into a gas separationtank, c. separating gas from said pulp in the gas separation tank toyield reduced gas pulp, d. feeding the reduced gas pulp into a head boxof the web formation apparatus, e. changing the feed of the reduced gaspulp to the head box as production changes in the web formationapparatus, wherein the change in the production of the web formationapparatus initiates a regulation system of the approach system, whichregulation system essentially simultaneously checks a need for changingan operational mode of the mixing pump, initiates a change of theoperational mode of the mixing pump according to said need, and bothguides and regulates a head box feed pump, wherein the regulation systemof the approach system controls both a pressure of the head box and asurface level of the gas separation tank, wherein a surface level of thegas separation tank is controlled by arranging a change of the head boxpressure to initiate a control function of the regulation system andwherein the regulation system controls the mixing pump anticipatorily inrelation to the feed pump so that the head box pressure and the surfacelevel in the gas separation tank remain constant.
 16. A method ofcontrolling the operation of an approach system of a web formationapparatus, in which said method comprises: a. forming a pulp from whitewater, fiber suspension and fillers, b. feeding said pulp by a mixingpump, into a gas separation tank, c. separating gas from said pulp inthe gas separation tank to yield reduced gas pulp, d. feeding thereduced gas pulp into a head box of the web formation apparatus, e.changing the feed of the reduced gas pulp to the head box as productionchanges in the web formation apparatus, wherein the change in theproduction of the web formation apparatus initiates a regulation systemof the approach system, which regulation system essentiallysimultaneously checks a need for changing an operational mode of themixing pump, initiates a change of the operational mode of the mixingpump according to said need, and both guides and regulates a head boxfeed pump, wherein in a grade change situation, a capacity of the mixingpump and a capacity of the feed pump are changed in a stepwise manner.17. A method of controlling, the operation of an approach system of aweb formation apparatus, in which said method comprises: a. forming apulp from white water, fiber suspension and fillers, b. feeding saidgulp by a mixing pump into a gas separation tank, c. separating gas fromsaid pulp in the gas separation tank, to yield reduced gas pulp, d.feeding the reduced gas pulp into a head box of the web formationapparatus, e. changing the feed of the reduced gas pulp to the head boxas production changes in the web formation apparatus, wherein the changein the production of the web formation apparatus initiates a regulationsystem of the approach system, which regulation system essentiallysimultaneously checks a need for changing an operational mode of themixing pump, initiates a change of the operational mode of the mixingpump according to said need, and both guides and regulates a head boxfeed pump, wherein the regulation system of the approach system controlsboth a pressure of the head box and a surface level of the gasseparation tank and wherein said surface level regulation is controlledby means of fuzzy logic.
 18. A method to provide and regulate pulpflowing through an approach system to a web formation apparatuscomprising: a. forming the pulp from white water, fiber suspension andfillers, b. feeding the pulp by a mixing pump into a gas separationtank, c. separating gas from the pulp in the tank to produce reduced gaspulp, d. feeding reduced gas pulp from the tank, through a head box feedpump and to a head box of the web formation apparatus, e. changing aproduction operational mode of the head box, and f. in anticipation ofthe change in the production operational mode, a regulation system forthe approach system determines a change to an operational pump mode ofthe mixing pump and implements the change in the operational pump modeby coordinated adjustments of an operation point of the mixing pump andan operating point of the head box feed pump.
 19. A method as in claim18 wherein the regulation system adjusts a pulp feed output of the headbox feed pump to maintain a constant pulp pressure at the head boxconstant.
 20. A method as in claim 18 wherein the regulation systemvaries a surface level in the gas separation tank by adjusting theoperating points of the mixing pump and the head box feed pump.
 21. Amethod as in claim 20 wherein the surface level in the gas separationtank is temporarily changed to without changing a feed rate to the headbox.